In order to earn this achievement you must complete the final battle of the first island on hard mode, this battle is know as the battle of Darkwater.
This game is very easy on any difficulty and will not put up much of a fight even on hard.
Each island is split up into two sections with one large battle taking place on both sides of the island. In order to unlock the first large battle you must complete every side quest in the first section of the first island. Once you have done this you will be given the option to initiate an attack on an area which is heavily fortified by legion.
Once you have completed the first large battle you will be given access to the second side of the first island. When you set foot on the second side of the first island a number of new side quests will be open to you, you must then complete these side quests in order to initiate the battle of Darkwater.
This is the first fight when you have access to dragons and need to eliminate shamans in order to win the battle.
If you find large battles difficult I suggest that you use the ice power up rather frequently. This power up allows you to cut through large amount of enemies without them being able to attack you. Additionally, if you can kill large numbers of enemies in a certain place your troops will follow you there and advance closer towards the shaman which you are trying to attack.
When you complete the battle of Darkwater the achievement should unlock. If this is your first time playing through the game you should also unlock the achievement named “Hero of Darkwater"